Results of an early clinical trial of the test appear in the latest issue of the journal Brain.

A total of eight patients with evidence of retinal neurodegeneration and eight healthy individuals took part in the study.

The Darc test enabled labelled cells to be identified up to six hours after injection.

A positive test result was significantly correlated with a diagnosis of glaucoma and later loss of vision.

The fluorescent marker, known as ANX776, was found to be safe and well-tolerated with no adverse side effects.

Chief investigator Professor Philip Bloom, from Western Eye Hospital, part of Imperial College Healthcare NHS Trust, said: "Treatment is much more successful when it is begun in early stages of the disease, when sight loss is minimal.

"Our developments mean we could diagnose patients 10 years earlier than was previously possible."

The scientists believe the test may have wider applications because the nerves that help us see are essentially an extension of the brain.

Programmed cell death, or apoptosis, at the back of the eye could therefore provide an early indication of neurodegenerative disease.

The research is funded by the Wellcome Trust charity.

Bethan Hughes, from Wellcome's Innovation team, said: "This innovation has the potential to transform lives for those who suffer loss of sight through glaucoma, and offers hope of a breakthrough in early diagnosis of other neurodegenerative diseases.

"Loss of sight as you age is an incredibly difficult disability, impacting quality of life and independence."